Use the Newsletter Subscription webpart, in which you would specify the Newsletter name.
For a more advanced version of the Newsletter Subscription web part have a look at http://www.mcbeev.com/Blog/November-2015/Free-Web-Part-for-Kentico-Better-Subscription-Form Note: In the 2nd screenshot of the web part configuration under Dialog Settings you can see a drop down list where you specify the Newsletter name.
To make my answer more complete, if you are not using Kentico for Newsletters then you can create a new form and then use the On-line form web part to display that form. Although I would recommend you go the route of using Kentico for Newsletters.